This invention relates to polyurethanes and, more particularly, to preparation of polyurethanes using new catalysts.
Polyurethanes are made by a wide variety of processes differing in detail. However, the basic urethane-forming reaction is the result of contact between an active-hydrogen containing compound, frequently a polyol, i.e., a dihydroxy or polyhydroxy compound, and a diisocyanate or polyisocyanate. The reaction of these starting materials normally requires the presence of a catalyst. A number of catalysts for this purpose are known. Among those most frequently used are tertiary amines, such as, for example, triethylene diamine and N-substitute morpholines; tin(II) salts of organic acids, such as for example tin(II); and heavy metals, such as mercury.
In the case of the tertiary amines and tin(II) salts, the catalysts serve to immediately or almost-immediately promote the reaction between the starting materials and thus may perform satisfactorily where processing requires such rapid initiation of the reaction. For some purposes, however, it is desirable to delay the reaction and therefore lengthen the time between contact of the components and gelation, thereby achieving greater processing latitude. For these purposes it is desirable to employ alternative, so-called "delayed action" catalysts such as the heavy metals.
However, the mentioned catalysts exhibit certain disadvantages that may limit their use. The amines and tin salts may result in premature gelation where processing requires additional time following contact between the starting materials. Such may be the case in the preparation of certain polyurethanes such as foams, elastomers, coatings and adhesives, where the formulation components are mixed and then the mixture is poured into a mold or onto a substrate and then adequately dispersed before gelation desirably occurs. Catalysts containing heavy metals, such as mercury, bismuth, barium or cadmium, may present toxicity and environmental safety problems that are difficult to overcome.
One method of preventing premature gelation without relying on the heavy metals is disclosed in U.S. Pat. No. 3,661,885 to Haddick. That invention is drawn to use of a preformed complex of a tin(II) salt and an organic complexing agent. The mentioned tin salts include stannous chloride and salts of organic acids, e.g., aliphatic carboxylic acids such as stannous acetate, oxalate, or octanoate, or a mixture of branched aliphatic monocarboxylic acids containing from 9 to 11 carbon atoms. Complexing with secondary and tertiary amines is preferred. The use of the complex delays catalysis until an "induction period" has elapsed. However, the tin(II) salt/amine complexes tend to decompose in the presence of water, which results in loss of catalytic activity. Furthermore, the delay may be insufficient to allow for optimum processing and product quality. Thus, their applicability is somewhat limited.
Other means of preventing premature gelation include the use of "acid-blocked catalysts", such as N-hydroxyalkyl quaternary ammonium carboxylate and other amine compounds blocked with acids such as formic acid.